查询的意思就是查找，寻找，指在某一个或几个地方找出自己所要的信息，假如我想搜索一下我自己写的博客，名字叫做初雪之恋，我在百度的搜索框中输入丁国华三个字，会有怎样的惊喜等着我？

啊哦，这个信息并不是我想要的，于是我再搜，“丁国华 廊坊师范学院  csdn”搜索结果如下：

很显然，这个还不是我想要的结果，我接着搜，“丁国华 廊坊师范学院 csdn 初雪之恋”；

在我的理解中，组合查询就是通过条件层层限制，一步一步精确，得到我们想要的答案，在机房收费系统中，有好几处涉及组合查询，接下来，以学生基本信息维护为例，讲解组合查询。在第二次机房收费系统中，我没有按原来胡阳版的模板来，原版的通过卡号，学号，姓名，性别，系别，年级，班号七个字段，来进行查询，操作符分别为等于，大于，小于，不等于，可是，现实生活中哪有性别大于什么，姓名小于什么的条件，所以我重新设计了一下窗体，从中挑选了四个字段名作为组合查询的条件简化了组合查询的难度，主界面设计如下：

接下来，看看我们的具体实现过程，首先实体层---E层；

<span style="font-size:18px;">Public Class Studentinfo
    Public studentNo As String                      '字段
    Public Property _studentNo As String            '属性
        Get
            Return studentNo
        End Get
        Set(value As String)
            studentNo = value
        End Set
    End Property
    Public cardNo As String
    Public Property _cardNo As String
        Get
            Return cardNo
        End Get
        Set(value As String)
            cardNo = value
        End Set
    End Property
    Public studentName As String
    Public Property _studentName As String
        Get
            Return studentName
        End Get
        Set(value As String)
            studentName = value
        End Set
    End Property
    Public sex As String
    Public Property _sex As String
        Get
            Return sex
        End Get
        Set(value As String)
            sex = value
        End Set
    End Property
    Public department As String
    Public Property _department
        Get
            Return department
        End Get
        Set(value)
            department = value
        End Set
    End Property
    Public grade As String
    Public Property _grade
        Get
            Return grade
        End Get
        Set(value)
            grade = value
        End Set
    End Property
    Public stuclass As String
    Public Property _stuclass
        Get
            Return stuclass
        End Get
        Set(value)
            stuclass = value
        End Set
    End Property
    Public remark As String
    Public Property _remark
        Get
            Return remark
        End Get
        Set(value)
            remark = value
        End Set
    End Property
    Public Handler As String
    Public Property _Handler
        Get
            Return Handler
        End Get
        Set(value)
            Handler = value
        End Set
    End Property
End Class
</span>

数据访问层---D层

<span style="font-size:18px;">Imports System.Data.SqlClient
Public Class StuBaInforPreserveDAO
    Public Function query(ByVal cmbOperator1 As String, ByVal cardNo As String, ByVal cmbOperator2 As String, ByVal studentNo As String, ByVal cmbOperator3 As String, ByVal department As String, ByVal cmbOperator4 As String, ByVal grade As String) As DataTable
        Dim db As New Entity.Dbutil '实例化数据库连接
        Dim dt As New DataTable       '实例化DataTable

        Using conn As New SqlConnection(db.connstring)
            conn.Open()                       '打开数据库连接

            Dim sql As String
            Dim cmd As SqlCommand
            Dim dataAdpter As New SqlDataAdapter
            Dim dst As New DataSet
            Dim flag(3) As Boolean             ’定义标签

            sql = "select studentNo,cardNo,studentName,sex,department,grade,class,remark,handler  from Student_info where "
            If Not (cardNo = "") Then                                   '如果卡号不为空，执行下面这句sql语句
                sql = sql & "studentNo" & cmbOperator1 & "'" & cardNo & "'"
                flag(0) = True
            End If

            If Not (studentNo = "") Then     '如果学号不为空，且卡号不为空，执行第一个sql语句，如果学号不为空，卡号为空执行第二个sql语句
                flag(1) = True
                If flag(0) = True Then
                    sql = sql & "and studentNo " & cmbOperator2 & "'" & studentNo & "'"
                Else
                    sql = sql & "studentNo" & cmbOperator2 & "'" & studentNo & "'"
                End If
            End If

            If Not (department = "") Then '如果系别不为空，且学号和卡号中有一个被选择，或者两个都选，执行第一个sql语句，否则执行第二个sql语句
                flag(2) = True
                If flag(0) = True Or flag(1) = True Then
                    sql = sql & "and department " & cmbOperator3 & "'" & department & "'"
                Else
                    sql = sql & "department" & cmbOperator3 & "'" & department & "'"
                End If
            End If

            If Not (grade = "") Then      '如果年级不为空，且学号，卡号，系别中有一个被选择，或者一个or两个被选择执行第一个sql语句，否则执行第二句
                flag(3) = True
                If flag(0) = True Or flag(1) = True Or flag(2) = True Then
                    sql = sql & "and grade" & cmbOperator4 & "'" & grade & "'"
                Else
                    sql = sql & "grade" & cmbOperator4 & "'" & grade & "'"
                End If
            End If
            cmd = New SqlCommand(sql, conn)
            dataAdpter.SelectCommand = cmd
            dataAdpter.Fill(dst, "Student_info")
            dt = dst.Tables("Student_info")
        End Using
        Return dt
    End Function
End Class
</span>

业务逻辑层---B层；

<span style="font-size:18px;">Public Class StuBaInforPreserveManager
    Public Function query(ByVal cmbOperator1 As String, ByVal cardNo As String, ByVal cmbOperator2 As String, ByVal studentNo As String, ByVal cmbOperator3 As String, ByVal department As String, ByVal cmbOperator4 As String, ByVal grade As String) As DataTable
        Dim StuBaInforPreserveDAO As New DAL.StuBaInforPreserveDAO                 '调用D层
        Return StuBaInforPreserveDAO.query(cmbOperator1, cardNo, cmbOperator2, studentNo, cmbOperator3, department, cmbOperator4, grade)
    End Function
End Class</span>

显示层---U层；

<span style="font-size:18px;">Imports BLL

Public Class frmStuBaInforPreserve
    Private Sub btnInquery_Click(sender As Object, e As EventArgs) Handles btnInquery.Click
        Dim StuBaInforPreserveManager As New BLL.StuBaInforPreserveManager  '调用B层
        Dim dt As New DataTable

        dt = StuBaInforPreserveManager.query(cmbOperator1.Text, txtCardNo.Text, cmbOperator2.Text, txtStuNo.Text, cmbOperator3.Text, txtDepartment.Text, cmbOperator4.Text, txtGrade.Text)
        DataGridView1.AutoGenerateColumns = True  '自动创建列
        DataGridView1.DataSource = dt

        DataGridView1.Columns(0).HeaderText = "学号"         '把相关信息在datagridview中显示出来
        DataGridView1.Columns(1).HeaderText = "卡号"
        DataGridView1.Columns(2).HeaderText = "姓名"
        DataGridView1.Columns(3).HeaderText = "性别"
        DataGridView1.Columns(4).HeaderText = "系别"
        DataGridView1.Columns(5).HeaderText = "年级"
        DataGridView1.Columns(6).HeaderText = "班级"
        DataGridView1.Columns(7).HeaderText = "备注"

    End Sub

    Private Sub frmStuBaInforPreserve_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        cmbOperator1.Enabled = False             '窗体加载时，需要注意的一些细节问题，比如那些按钮能点，那些不能点
        txtCardNo.Text = ""
        txtCardNo.Enabled = False

        cmbOperator1.Items.Add("=")
        cmbOperator1.Items.Add(">")
        cmbOperator1.Items.Add("<")
        cmbOperator1.Items.Add("<>")
        cmbOperator1.SelectedIndex = 0

        cmbOperator2.Enabled = False
        txtStuNo.Text = ""
        txtStuNo.Enabled = False
        cmbOperator2.Items.Add("=")
        cmbOperator2.Items.Add(">")
        cmbOperator2.Items.Add("<")
        cmbOperator2.Items.Add("<>")
        cmbOperator2.SelectedIndex = 0

        cmbOperator3.Enabled = False
        txtDepartment.Text = ""
        txtDepartment.Enabled = False
        cmbOperator3.Items.Add("=")
        cmbOperator3.Items.Add("<>")
        cmbOperator3.SelectedIndex = 0

        cmbOperator4.Enabled = False
        txtGrade.Text = ""
        txtGrade.Enabled = False
        cmbOperator4.Items.Add("=")
        cmbOperator4.Items.Add("<>")
        cmbOperator4.SelectedIndex = 0

    End Sub

    Private Sub btnExit_Click(sender As Object, e As EventArgs) Handles btnExit.Click        '退出按钮
        Me.Close()
    End Sub
    Private Sub chkCardNo_Click(sender As Object, e As EventArgs) Handles chkCardNo.Click  '如果卡号被选中，则txtCardNo和cmbOperator1可以点否者不能点，并且文本框的内容自动情况，恢复默认值
        If chkCardNo.Checked Then
            txtCardNo.Enabled = True
            cmbOperator1.Enabled = True
        Else
            txtCardNo.Enabled = False
            cmbOperator1.Enabled = False
            txtCardNo.Text = ""
            cmbOperator1.SelectedIndex = 0

        End If
    End Sub
    Private Sub chkStuNo_Click(sender As Object, e As EventArgs) Handles chkStuNo.Click
        If chkStuNo.Checked Then
            txtStuNo.Enabled = True
            cmbOperator2.Enabled = True
        Else
            txtStuNo.Enabled = False
            cmbOperator2.Enabled = False
            txtStuNo.Text = ""
            cmbOperator2.SelectedIndex = 0
        End If
    End Sub

    Private Sub chkDepartment_Click(sender As Object, e As EventArgs) Handles chkDepartment.Click
        If chkDepartment.Checked Then
            txtDepartment.Enabled = True
            cmbOperator3.Enabled = True
        Else
            txtDepartment.Enabled = False
            cmbOperator3.Enabled = False
            txtDepartment.Text = ""
            cmbOperator3.SelectedIndex = 0
        End If
    End Sub
    Private Sub chkGrade_Click(sender As Object, e As EventArgs) Handles chkGrade.Click
        If chkGrade.Checked Then
            txtGrade.Enabled = True
            cmbOperator4.Enabled = True
        Else
            txtGrade.Enabled = False
            cmbOperator4.Enabled = False
            txtGrade.Text = ""
            cmbOperator4.SelectedIndex = 0
        End If
    End Sub
End Class</span>

运行效果如下：

自此学生基本信息维护就暂时告一段落，开始敲学生基本信息维护的时候，不知道从哪儿下手，各种纠结，各种逃避，各种不想敲，可是，我已经长大了，我不能再由着我自己的性子来做事，我要为我自己做的事情负责任，走过去了，真的有那么困难么？我想其中的滋味只有走过的人才能够真正明白。

曾经看到米老师给一篇博文这样的评语，这篇博客的主要内容，我记不清楚了，好像是解决一个问题，然后成功解决的一篇博文，米老师说，这个过程是不是很养人，对，这个过程不光养人，而且锻炼人，考验我们面对新知识的心态，是一拖再拖，今目标红了又红，还是坦然面对，两天把三个组合查询一一搞定，机房收费系统未完，待续......